Fouling plugs on my YZ 426 F

I bought an 01 YZ 426 a couple of months ago, and it has started to foul plugs rapidly. The bike runs great when it runs, but as soon as I stop it won't kick back up again.

The other day I took it out and rode for about two solid hours. After taking a break the bike wouldn't start, and the obvious answer was a fouled plug. I did a field cleaning because I didn't have a spare (I didn't have the spark plug wrench needed either, but thats it another story and a lesson learned).

I put the plug back in, and it kicked on the third try. I let it run for maybe five minutes or so and it fouled out again. I had changed the fuel, oil and cleaned the carb prior to this happening.

So what could be making my bike foul out so easily?

I'm going to pick up a new air filter tomorrow. I should have replaced it when I bought the bike.

Also the seals look really good. I'm very doubtful of oil leakage.

I'm also going to do some more cleaning, where the spark plug screws in. Its really dirty down there.

A common error is to use the wrong plug. The bike needs a CR8E, or equivalent heat range. When people make the mistake of thinking a colder plug will help the engine run colder/better and substitute a CR9E, the bike will foul them left and right, because they aren't hot enough to keep themselves clean.

As to your spark plug, it may well just be worn out. Old plugs are easily fouled, and not easily cleaned. That said, the bike needs to be running right in order to keep the plug clean, too.

BTW, the air filter is intended to be cleaned in solvent or a strong detergent, dried and then to have air filter oil worked into it no less frequently than each second or third ride. They generally don't need to be replaced. The oil is important, because foam filters don't filter without it.

Thanks for the input on the CR9E spark plugs. That is what the original owner was using, but I have been using CR8E's. I will pick up a few of those.

I was having the same problem with mine a few years ago. Turns out the fuel petcock was leaking. If the bike was warm I was fine, but as soon as I let it sit for 5 or 10 minutes I was stuck. New seal and bent the tension ring a little and havent had a problem since.

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